Capacitively coupled plasma reactor with magnetic plasma control

ABSTRACT

A plasma reactor includes a vacuum enclosure including a side wall and a ceiling defining a vacuum chamber, and a workpiece support within the chamber and facing the ceiling for supporting a planar workpiece, the workpiece support and the ceiling together defining a processing region between the workpiece support and the ceiling. Process gas inlets furnish a process gas into the chamber. A plasma source power electrode is connected to an RF power generator for capacitively coupling plasma source power into the chamber for maintaining a plasma within the chamber. The reactor further includes at least a first overhead solenoidal electromagnet adjacent the ceiling, the overhead solenoidal electromagnet, the ceiling, the side wall and the workpiece support being located along a common axis of symmetry.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/841,116, filed May 7, 2004 entitled CAPACITIVELY COUPLED PLASMAREACTOR WITH MAGNETIC PLASMA CONTROL, by Daniel Hoffman, et al., whichis a divisional of U.S. patent application Ser. No. 10/192,271, filedJul. 9, 2002 entitled CAPACITIVELY COUPLED PLASMA REACTOR WITH MAGNETICPLASMA CONTROL, by Daniel Hoffman, et al., issued as U.S. Pat. No.6,853,141 on Feb. 8, 2005, which claims priority of U.S. ProvisionalApplication Ser. No. 60/638,194, filed May 22, 2002 entitledCAPACITIVELY COUPLED PLASMA REACTOR WITH MAGNETIC PLASMA CONTROL byDaniel Hoffman, et al. This application is also a continuation-in-partof U.S. patent application Ser. No. 11/105,307, filed Apr. 12, 2005entitled MERIE PLASMA REACTOR WITH OVERHEAD RF ELECTRODE TUNED TO THEPLASMA WITH ARCING SUPPRESSION, by Daniel Hoffman, et al., issued asU.S. Pat. No. 7,186,943 on Mar. 6, 2007, which is a continuation of U.S.patent application Ser. No. 10/007,367, filed Oct. 22, 2001 entitledMERIE PLASMA REACTOR WITH OVERHEAD RF ELECTRODE TUNED TO THE PLASMA WITHARCING SUPPRESSION, by Daniel Hoffman, et al., issued as U.S. Pat. No.6,894,245 on May 17, 2005, which is a continuation-in-part of U.S.patent application Ser. No. 09/527,342, filed Mar. 17, 2000 entitledPLASMA REACTOR WITH OVERHEAD RF ELECTRODE TUNED TO THE PLASMA by DanielHoffman, et al., issued as U.S. Pat. No. 6,528,751 on Mar. 4, 2003, allof which are assigned to the present assignee.

BACKGROUND

Capacitively coupled plasma reactors are used in fabricatingsemiconductor microelectronic structures with high aspect ratios. Suchstructures typically have narrow, deep openings through one or more thinfilms formed on a semiconductor substrate. Capacitively coupled plasmareactors are used in various types of processes in fabricating suchdevices, including dielectric etch processes, metal etch processes,chemical vapor deposition and others. Such reactors are also employed infabricating photolithographic masks and in fabricating semiconductorflat panel displays. Such applications depend upon plasma ions toenhance or enable desired processes. The density of the plasma ions overthe surface of the semiconductor workpiece affects the processparameters, and is particularly critical in the fabrication of highaspect ratio microelectronic structures. In fact, a problem infabricating high aspect ratio microelectronic integrated circuits isthat non-uniformities in the plasma ion density across the workpiecesurface can lead to process failure due to non-uniform etch rates ordeposition rates.

A typical capacitively coupled reactor has a wafer support pedestal inthe reactor chamber and a ceiling overlying the wafer support. Theceiling may include a gas distribution plate that sprays process gasinto the chamber. An RF power source is applied across the wafer supportand ceiling or wall to strike and maintain a plasma over the wafersupport. The chamber is generally cylindrical, while the ceiling andwafer support are circular and coaxial with the cylindrical chamber toenhance uniform processing. Nevertheless, such reactors have non-uniformplasma density distributions. Typically, the radial density distributionof plasma ions is high over the center of the wafer support and low nearthe periphery, a significant problem. Various approaches are used tocontrol the plasma ion density distribution so as to improve processuniformity across the wafer or workpiece surface, and at least partiallyovercome this problem.

One such approach is to provide a set of magnetic coils spacedcircumferentially around the side of the reactor chamber, the coils allfacing the center of the chamber. A relatively low frequency sinusoidalcurrent is supplied to each coil, the sinusoidal currents in adjacentcoils being offset in phase so as to produce a slowly rotating magneticfield over the wafer support. This feature tends to improve the radialdistribution of plasma ion density over the wafer support. Where thisapproach is employed in reactive ion etching, it is called magneticallyenhanced reactive ion etching (MERIE). This approach has certainlimitations. In particular, the strength of the magnetic field may needto be limited in order to avoid device damage to microelectronicstructures on the semiconductor workpiece associated with the strengthof the magnetic field. The strength must also be limited to avoidchamber arcing associated with the rate of change of magnetic fieldstrength. As a result, the total MERIE magnetic field may need to besubstantially reduced and therefore may face substantial limitations inplasma ion density uniformity control.

Another approach is called configurable magnetic fields (CMF) andemploys the same circumferentially spaced coils referred to above. But,in CMF the coils are operated so as to impose a magnetic field thatextends across the plane of the workpiece support, from one side to theother. In addition, the magnetic field rotates about the axis of thewafer support, to produce a time-averaged magnetic field that is radial.This is all accomplished, in the case of a reactor having fourside-by-side coils, by furnishing one D.C. current to one pair ofadjacent coils and a different (or opposite) D.C. current to theopposite pair of adjacent coils. The coils are switched to rotate thispattern so that the magnetic field rotates, as mentioned above. Thisapproach is vulnerable to chamber or wafer arcing problems due to theabrupt switching of the CMF magnetic fields, and therefore the magneticfield strength must be limited. As a result, in some applications themagnetic field cannot be sufficient to compensate for plasma ion densitynon-uniformities produced by the reactor.

Thus, what is needed is a way of compensating for plasma ion densitydistribution non-uniformities more efficiently (so that the magneticfield strength can be less) and with less (or with no) time fluctuationsin the magnetic field.

SUMMARY OF THE INVENTION

A plasma reactor includes a vacuum enclosure including a side wall and aceiling defining a vacuum chamber, and a workpiece support within thechamber and facing the ceiling for supporting a planar workpiece, theworkpiece support and the ceiling together defining a processing regionbetween the workpiece support and the ceiling. Process gas inletsfurnish a process gas into the chamber. A plasma source power electrodeis connected to an RF power generator for capacitively coupling plasmasource power into the chamber for maintaining a plasma within thechamber. The reactor further includes at least a first overheadsolenoidal electromagnet adjacent the ceiling, the overhead solenoidalelectromagnet, the ceiling, the side wall and the workpiece supportbeing located along a common axis of symmetry. A current source isconnected to the first solenoidal electromagnet and furnishes a firstelectric current in the first solenoidal electromagnet whereby togenerate within the chamber a magnetic field which is a function of thefirst electric current, the first electric current having a value suchthat the magnetic field increases uniformity of plasma ion densityradial distribution about the axis of symmetry near a surface of theworkpiece support.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C illustrate a plasma reactor with an overhead VHFelectrode and overhead coils for controlling plasma ion uniformity.

FIG. 2 illustrates an exemplary apparatus for controlling the overheadcoils of FIG. 1.

FIGS. 3A and 3B are graphical representations of a magnetic field of theoverhead coils of FIG. 1 and FIG. 3C is a spatial representation of thesame field.

FIGS. 4A, 4B, 4C and 4D are graphs of the etch rate (vertical axis) onthe wafer surface as a function of radial location (horizontal axis) forvarious modes of operation of the reactor of FIG. 1.

FIGS. 5A, 5B, 5C and 5D are graphs of the etch rate (vertical axis) onthe wafer surface as a function of radial location (horizontal axis) forfurther modes of operation of the reactor of FIG. 1.

FIG. 6 is a graph depicting etch rate as a function of magnetic field.

FIGS. 7 and 8 illustrate the reactor of FIG. 1A with MERIE magnets.

FIG. 9 depicts a method of operating the reactor of FIG. 1A.

FIG. 10 is a graph illustrating a comparative example of magneticpressure and ion or electron density as functions of radial location onthe wafer surface in the reactor of FIG. 1A.

FIG. 11 is a graph depicting etch rate non-uniformity as a function ofcoil current.

FIG. 12 illustrates radial ion distribution at zero coil current in theexample of FIG. 11.

FIGS. 13A and 13B compare measured and predicted etch rate distributionsat a coil current of about 11 amperes in the example of FIG. 11.

FIGS. 14A and 14B compare measured and predicted etch rate distributionsat a coil current of about 35 amperes in the example of FIG. 11.

FIG. 15 depicts a further method of operating the reactor of FIG. 1A.

FIG. 16 illustrates a magnetic field distribution obtained in a reactorcorresponding to FIG. 1A.

FIG. 17 depicts the gradient of the square of the magnetic field of FIG.16 in the wafer plane.

FIG. 18 illustrates another magnetic field distribution obtained in areactor corresponding to FIG. 1A.

FIG. 19 depicts the gradient of the square of the magnetic field of FIG.18 in the wafer plane.

FIG. 20 illustrates a yet further magnetic field distribution obtainedin a reactor corresponding to FIG. 1A.

FIG. 21 depicts the gradient of the square of the magnetic field of FIG.20 in the wafer plane.

FIG. 22 depicts yet another method of operating the reactor of FIG. 1A.

FIG. 23 illustrates an exemplary microcontroller operation forcontrolling the reactor of FIG. 1A.

FIG. 24 illustrates a plasma reactor including features contained in thereactor of FIG. 1A.

FIG. 25 illustrates another plasma reactor including features containedin the reactor of FIG. 1A.

FIGS. 26, 27, 28, 29A and 29B illustrate a gas distribution plate forthe reactors of FIGS. 1A, 24 and 25.

FIGS. 30 and 31 illustrate thermal control features in gas distributionplate like that of FIG. 26.

FIGS. 32 and 33 illustrate a gas distribution plate corresponding toFIG. 26 having dual zone gas flow control.

FIG. 34 illustrates a plasma reactor corresponding to FIG. 1A having thedual zone gas distribution plate.

FIGS. 35 and 36 illustrate exemplary dual zone gas flow controllers.

FIG. 37 illustrates a plasma reactor corresponding to FIG. 34 havingthree overhead coils for controlling plasma ion distribution.

FIGS. 38 and 39 depict different gas injection hole patterns in the gasdistribution plate of FIG. 26 for producing center low or center highgas flow distributions, respectively.

FIGS. 40, 41, 42 and 43 illustrate different arrangements of overheadcoils for controlling plasma ion distribution.

FIGS. 44 and 45 illustrate a plasma reactor corresponding to FIG. 1A inwhich the overhead coils are replaced by upper and lower magnetic coilsabove and below the reactor chamber to produce a cusp-shaped magneticfield best seen in FIG. 45.

FIG. 46 illustrates how the upper and lower coils of FIG. 44 can bereplaced by configurable magnetic field (CMF) coils operated in such amanner as to produce the cusp-shaped magnetic field of FIG. 45.

FIGS. 47A-47D illustrate a mode of operation of the CMF coils of FIG. 46to produce a desired magnetic field configuration.

FIGS. 48, 49 and 50 illustrate an annular apertured plate in the reactorof FIG. 1A for preventing plasma ions from entering the reactor'spumping annulus.

FIG. 51 illustrates a rectangular version of the reactor of FIG. 1A forprocessing rectangularly shaped workpieces.

FIG. 52 illustrates a reactor corresponding to FIG. 1A having aretractable workpiece support pedestal.

DETAILED DESCRIPTION

The plasma ion density distribution exhibited by a particular plasmareactor is a function of chamber pressure, gas mixture and diffusion,and source power radiation pattern. In the present invention, thisdistribution is magnetically altered to approximate a selected or idealdistribution that has been predetermined to improve process uniformity.The magnetically altered or corrected plasma ion density distribution issuch that process uniformity across the surface of the wafer orworkpiece is improved. For this purpose, the magnetically correctedplasma distribution may be non-uniform or it may be uniform, dependingupon the needs determined by the user. We have discovered that theefficiency with which an average magnetic field strength exerts pressureon a plasma to change its distribution to a desired one can be improved.This surprising result can be achieved in accordance with this discoveryby increasing the radial component of the gradient of the magneticfield. The radial direction is understood to be about the axis ofsymmetry of the cylindrical chamber. Thus, what is needed is a magneticfield configuration which has a large radial gradient and a small fieldstrength in other directions. Such a magnetic field is cusp-shaped withits axis of symmetry coinciding with the axis of the cylindrical reactorchamber. One way of producing a cusp-shaped magnetic field is to providecoils above and below the cylindrical chamber and run D.C. currentsthrough these coils in opposite directions.

Depending upon the chamber design, it may be impractical to provide acoil below the wafer pedestal, and therefore in a first case, a top coilsuffices for these purposes. In addition, what is needed is for thecusp-shaped magnetic field to be configurable or adjustable for accuratecontrol or alteration of a plasma ion distribution inherent in a givenplasma reactor chamber (the “ambient” plasma ion distribution). Sincethe plasma ion distribution provided in different capacitively coupledreactors can vary widely, such adjustability may be essential in somecases. The radial component of the magnetic field gradient is chosen toapply the magnetic pressure required to alter the ambient distributionto the desired distribution. For example, if the desired distribution isa uniform distribution, then the applied magnetic field is selected tocounteract the non-uniformity in the radial distribution of plasma iondensity exhibited by the reactor in the absence of the magnetic field.In this case, for example, if the reactor tends to have a center-highdistribution of plasma ion density, then the magnetic field gradient ischosen to sustain the plasma density over the center of the wafersupport pedestal and enhance it near the periphery to achieveuniformity.

Such adjustability of the cusp-shaped magnetic field is achieved inaccordance with our discovery by providing at least a second overheadcoil of a different (e.g., smaller) diameter than the first coil. TheD.C. currents in the respective coils are independently adjustable so asto permit configuration of the cusp-shaped magnetic field in a highlyflexible manner to alter virtually any ambient plasma ion distributionto approximate some desired plasma ion distribution. This choice offield configuration can be designed to modify center-high or center-lowplasma ion density distributions.

One advantage that can be realized is two-fold, in that the cusp-shapedmagnetic field has a large radial gradient relative to the magneticfield strength (as noted above) and therefore is highly efficient inexerting corrective pressure on the plasma; but, since the magneticfield is constant over time, there is far less tendency to producearcing, and therefore a somewhat stronger magnetic field may be employedfor even greater corrective capacity when required. As will be describedlater in this specification, this feature can be quite helpful at higherchamber pressures.

FIG. 1A illustrates a capacitively coupled plasma reactor capable ofproviding an adjustable cusp-shaped magnetic field. The reactor of FIG.1A includes a cylindrical side wall 5, a ceiling 10 that is a gasdistribution plate, and a wafer support pedestal 15 that holds asemiconductor workpiece 20. The ceiling 10 or gas distribution plate maybe conductive so as to enable it to serve as an anode or it may have ananode attached to it. The ceiling 10 or gas distribution plate istypically made of aluminum and has an internal gas manifold and gasinjection orifices in its interior surface that face into the chamber. Aprocess gas supply 25 furnishes process gas to the gas distributionplate 10. A vacuum pump 30 controls the pressure inside the reactorchamber. Plasma source power for igniting and maintaining a plasmainside the reactor chamber is produced by an RF generator 40 connectedthrough an impedance match circuit 45 to the wafer support pedestal 15so that the wafer support pedestal serves as an RF electrode. The anode(which may be the ceiling 10 formed of a conductor material) isconnected to RF ground so that is serves as the counter electrode. Sucha reactor tends to have a very non-uniform plasma ion densitydistribution, which is typically center-high.

FIG. 1B illustrates a feature in which the ceiling 10, rather than beingconnected directly to ground as in FIG. 1A, is connected through an RFimpedance match element 11 (shown only schematically) to a VHF signalgenerator 12 that furnishes the plasma source power. In this case, theRF generator 40 merely controls the RF bias on the semiconductor waferor workpiece 20. (The RF impedance match element 11 may be a fixedtuning element such as for example a coaxial tuning stub or a strip linecircuit.) Such a feature is discussed in greater detail in a laterportion of this specification.

In order to control distribution of plasma ion density, a set ofinductive coils are provided above the ceiling 10. In the case of FIG.1A, the set of coils includes an inner coil 60 and an outer coil 65which are coaxial with the cylindrical chamber and each constitutessingle winding of a conductor. While the windings 60, 65 are illustratedin FIG. 1A as being single turns, they may each consist of plural turnsarranged vertically, for example as shown in FIG. 1B. Or, as shown inFIG. 1C, the windings 60, 65 may extend both vertically andhorizontally. In the case of FIG. 1A, the inner coil 60 is locatedfarther above the ceiling 10 than the outer coil 65. However, in othercases this arrangement may be reversed, or the two coils 60, 65 may beat the same height above the ceiling 10.

In the case of FIGS. 1A and 1B, a controller 90 determines the magnitudeand polarity of currents flowing to the respective overhead coils 60, 65by controlling respective independent D.C. current supplies 70, 75 thatare connected to respective ones of the coils 60, 65. Referring now toFIG. 2, a case is illustrated in which the controller 90 governs theD.C. currents to the coils 60, 65 from a D.C. current supply 76 thatfurnished current through the controller 90, the controller 90 beingconnected to respective ones of the coils 60, 65. In either case, thecontroller 90 is capable of causing D.C. currents of differentpolarities and magnitudes to flow in different ones of the coils 60, 65.In the case of FIG. 2, the controller 90 includes a pair ofpotentiometers 82 a, 82 b that adjust the D.C. current applied to therespective coils 60, 65 and a pair of ganged switches 84 a, 84 b thatindependently determine the polarity of the D.C. current applied to eachof the coils 60, 65. A programmable device such as a microprocessor 91can be included in the controller 90 in order to intelligently governthe potentiometers 82 a, 82 b and the ganged switches 84 a, 84 b.

The arrangement of the two coils 60, 65 illustrated in FIGS. 1A, 1B and1C, in which the inner coil 60 is placed at a greater height above theceiling 10 than the outer coil 65, provides certain advantages.Specifically, the radial component of the magnetic field gradientprovided by either coil is, at least roughly, proportional to the radiusof the coil and inversely proportional to the axial displacement fromthe coil. Thus, the inner and outer coils 60, 65 will perform differentroles because of their different sizes and displacements: The outer coil65 will dominate across the entire surface of the wafer 20 because ofits greater radius and closer proximity to the wafer 20, while the innercoil 60 will have its greatest effect near the wafer center and can beregarded as a trim coil for finer adjustments or sculpting of themagnetic field. Other arrangements may be possible for realizing suchdifferential control by different coils which are of different radii andplaced at different displacements from the plasma. As will be describedlater in this specification with reference to certain working examples,different changes to the ambient plasma ion density distribution areobtained by selecting not only different magnitudes of the currentsflowing in the respective overhead coils (60, 65) but also by selectingdifferent polarities or directions of current flow for the differentoverhead coils.

FIG. 3A illustrates the radial (solid line) and azimuthal (dashed line)components of the magnetic field produced by the inner coil 60 as afunction of radial position on the wafer 20, in the case of FIG. 1A.FIG. 3B illustrates the radial (solid line) and azimuthal (dashed line)components of the magnetic field produced by the outer coil 65 as afunction of radial position on the wafer 20. The data illustrated inFIGS. 3A and 3B were obtained in an implementation in which the wafer 20was 300 mm in diameter, the inner coil 60 was 12 inches in diameter andplaced about 10 inches above the plasma, and the outer coil 65 was 22inches in diameter and placed about 6 inches above the plasma. FIG. 3Cis a simplified diagram of the half-cusp shaped magnetic field linepattern produced by the inner and outer overhead coils 60, 65.

The controller 90 of FIG. 2 can change the currents applied to therespective coils 60, 65 in order to adjust the magnetic field at thewafer surface and thereby change the spatial distribution of plasma iondensity. What will now be illustrated are the effects of differentmagnetic fields applied by different ones of the coils 60, 65, in orderto illustrate how profoundly the controller 90 can affect and improveplasma ion distribution in the chamber by changing these magneticfields. In the following examples, the spatial distribution of the etchrate across the wafer surface rather than the plasma ion distribution ismeasured directly. The etch rate distribution changes directly withchanges in the plasma ion distribution and therefore changes in one arereflected by changes in the other.

FIGS. 4A, 4B, 4C and 4D illustrate the beneficial effects realized usingthe inner coil 60 only at a low chamber pressure (30 mT). FIG. 4Aillustrates measured etch rate (vertical Z axis) as a function oflocation (horizontal X and Y axes) on the surface of the wafer 20. FIG.4A thus illustrates the spatial distribution of the etch rate in theplane of the wafer surface. The center-high non-uniformity of the etchrate distribution is clearly seen in FIG. 4A. FIG. 4A corresponds to thecase in which no magnetic field is applied, and therefore illustrates anon-uniform etch rate distribution that is inherent in the reactor andneeds correction. The etch rate has a standard deviation of 5.7% in thiscase. In FIGS. 4A-D and FIGS. 5A-D, the magnetic field strength will bedescribed as the axial field near the center of the wafer although it isto be understood that the radial field is the one that works on theradial distribution of plasma ion density to improve uniformity. Theaxial field is chosen in this description because it is more readilymeasured. The radial field at the edge of the wafer typically is aboutone third the axial field at this location.

FIG. 4B illustrates how the etch rate distribution changes when theinner coil 60 has been energized to generate a magnetic field of 9Gauss. The non-uniformity decreases to a standard deviation of 4.7%.

In FIG. 4C the magnetic field of the inner coil 60 has been increased to18 Gauss, and it can be seen that the peak at the center has beengreatly diminished, with the result that the etch rate standarddeviation across the wafer is reduced to 2.1%.

In FIG. 4D the magnetic field of the inner coil 60 has been furtherincreased to 27 Gauss, so that the center high pattern of FIG. 4A hasbeen nearly inverted to a center low pattern. The standard deviation ofthe etch rate across the wafer surface in the case of FIG. 4D was 5.0%.

FIGS. 5A, 5B, 5C and 5D illustrate the beneficial effects of using boththe coils 60, 65 at higher chamber pressures (200 mT). FIG. 5Acorresponds to FIG. 4A and depicts the center-high etch ratenon-uniformity of the reactor uncorrected by a magnetic field. In thiscase, the standard deviation of the etch rate across the wafer surfacewas 5.2%.

In FIG. 5B, the outer coil 65 has been energized to produce a 22 Gaussmagnetic field, which decreases somewhat the center peak in the etchrate distribution. In this case, the etch rate standard deviation hasbeen decreased to 3.5%.

In FIG. 5C, both coils 60, 65 are energized to produce a 24 Gaussmagnetic field. The result seen in FIG. 5C is that the center peak inthe etch rate distribution has been significantly decreased, while theetch rate near the periphery has increased. The overall effect is a moreuniform etch rate distribution with a low standard deviation of 3.2%.

In FIG. 5D, both coils are energized to produce a 40 Guass magneticfield, producing an over-correction, so that the etch rate distributionacross the wafer surface has been transformed to a center-lowdistribution. The etch rate standard deviation in this latter case hasrisen slightly (relative to the case of FIG. 5C) to 3.5%.

Comparing the results obtained in the low pressure tests of FIGS. 4A-4Dwith the high pressure tests of FIGS. 5A-5D, it is seen that the higherchamber pressure requires a much greater magnetic field to achieve asimilar correction to etch rate non-uniform distribution. For example,at 30 mT an optimum correction was obtained using only the inner coil 60at 18 Gauss, whereas at 300 mT a magnetic field of 24 Gauss using bothcoils 60, 65 was required to achieve an optimum correction.

FIG. 6 shows that the magnetic fields of the overhead coils greatlyaffect the uniformity of plasma ion density or etch rate distribution,but do not greatly affect etch rate itself. This is an advantagebecause, while it is desirable to improve uniformity of etch ratedistribution, it is preferable to not change the etch rate chosen for aparticular semiconductor process. In FIG. 6, the diamond symbols depictmeasured etch rate (left-hand vertical axis) as a function of magneticfield (horizontal axis), while the square symbols depict standarddeviation (non-uniformity) of the etch rate (right-hand vertical scale)as a function of the magnetic field. The change in non-uniformity overthe illustrated range is about one order of magnitude, the change inetch rate is only about 25%.

The overhead coil inductors 60, 65 of FIGS. 1A, 1B and 1C may be usedwith a conventional MERIE reactor. FIGS. 7 and 8 illustrate an casecorresponding to FIG. 1A with the additional feature of fourconventional MERIE electromagnets 92, 94, 96, 98 and an MERIE currentcontroller 99. The current controller 99 provides A.C. currents to therespective MERIE electromagnets 92, 94, 96, 98. The respective currentsare of the same low frequency but have their phases offset by 90 degreesso as to produce a slowly rotating magnetic field within the chamber inthe conventional way.

Controlling Plasma Distribution with the Overhead Coils:

In accordance with a method of the invention, plasma ion densitydistribution across the wafer surface that is inherent in a particularreactor is tailored in a particular way by selecting a particular themagnetic field produced by the overhead coils 60, 65. For example, theplasma distribution may be tailored to produce a more uniform etch ratedistribution across the wafer surface. This tailoring is accomplished,for example, by programming the controller 90 to select optimumpolarities and amplitudes of the D.C. current flow in the overheadcoils. While the present example concerns a reactor with only twoconcentric overhead coils (i.e., the coils 60 and 65), the method can becarried out with more than two coils, and may provide more accurateresults with a greater number of overhead coils. The magnetic field istailored by the controller 90 to change the plasma ion densitydistribution across the wafer surface, which in turn affects the etchrate distribution.

A first step is to measure the etch rate distribution across the wafersurface in the absence of any corrective magnetic field from theoverhead coils 60, 65. A next step is to determine a change in theplasma ion density distribution that renders the etch rate distributionmore uniform. A final step is to determine a magnetic field that wouldproduce the desired change in plasma ion density distribution. Giventhis magnetic field, the magnitudes and directions of the currents inthe overhead coils 60, 65 necessary to produce such a field can becomputed from well-known static magnetic field equations.

We have found a way of computing, from the magnetic field, pressureexerted by the magnetic field of the overhead coils 60, 65 on the plasma(the so-called “magnetic pressure”). This will be discussed below. Themagnetic pressure on the plasma produces a change in plasma ion densitydistribution. This change in plasma ion density distribution produces aproportional change in etch rate distribution across the wafer surface,which can be directly observed. The plasma ion density distributionacross the wafer surface and the etch rate distribution are therefore atleast roughly related by a factor of proportionality.

Initially, the spatial distribution of the etch rate across the wafersurface is measured prior to the application of magnetic fields from theoverhead coils 60, 65. From this, a desired change in etch ratedistribution (to achieve a uniform distribution) can be determined.Next, the spatial distribution of the magnetic field produced by eachoverhead coil 60, 65 as a function of location within the chamber andcurrent flow in the coil is determined analytically from the geometry ofeach coil. Then, by applying a known set of currents to the coils andthen measuring the resulting change in etch rate distribution across thewafer surface, a linear scale factor can be deduced that relates thevector sum of the magnetic fields from all the coils at the wafersurface to the change in etch rate distribution at the wafer surface.(This scale factor is generally a function of neutral pressure in theplasma and is operative up to about 500 mT chamber pressure.) Therefore,given a desired change or correction in etch rate distribution (toachieve better uniformity), the necessary magnetic fields can be found(in a manner described later in this specification), and thecorresponding coil currents can be inferred therefrom using the magneticfield spatial distribution function previously determined analytically.

The desired correction to the non-uniformity in etch rate distributioncan be established in a variety of ways. For example, the 2-dimensionaletch rate distribution across the wafer surface can be subtracted from auniform or average etch rate to produce a “difference” distribution. Thenon-uniformities in etch rate distribution to be corrected in thismethod are the result of various factors in the reactor chamber,including non-uniform application of the capacitively coupled sourcepower, non-uniform process gas distribution as well as non-uniformplasma ion density distribution. In the foregoing method, thenon-uniformities are corrected by changing the plasma ion densitydistribution by magnetic pressure.

The following method can also be employed to establish a “corrected”plasma distribution that is non-uniform in some desired way. In thiscase, the correction to be made is the difference between the“uncorrected” or ambient plasma ion density distribution and the desireddistribution (that is itself non-uniform). Thus, the method is usefulfor making the plasma density distribution either more uniform or of aparticular selected density distribution pattern that is not necessarilyuniform.

A series of steps for carrying out the foregoing method will now bedescribed with reference to FIG. 9.

The first step (block 910 of FIG. 9) is to analytically determine, foreach one of the overhead coils 60, 65, the expression for the magneticfield at the wafer surface as a function of current flow in the coil andradial location on the wafer surface. Using cylindrical coordinates,this expression may be written, for the i^(th) coil, as B_(i)(r,z=wafer, I_(i)). It is determined from the Biot-Savart law in a verystraight-forward manner.

The next step (block 920 of FIG. 9) is carried out with no currentflowing in the overhead coils 60, 65. In this step, the spatialdistribution of plasma ion density across the wafer surface is measured.This spatial distribution may be written as n(r, z=wafer). In this step,the plasma ion density distribution can be measured indirectly bymeasuring the etch rate distribution across the surface of a test wafer.The skilled worker can readily infer the plasma ion density distributionfrom the etch rate distribution.

Next, in the step of block 930, a correction, c(r), to the measuredplasma ion density spatial distribution function n(r, z=wafer) measuredin the previous step is determined. The correction c(r) may be definedin any number of appropriate ways. For example, it may be defined as themaximum value n(r, z=wafer)_(max) minus n(r, z=wafer). In this way,adding c(r) to n(r, z=wafer) produces a “corrected” distribution with auniform amplitude equal to n(r)_(max). Of course, the correctionfunction c(r) may be defined differently to produce a different uniformamplitude. Or, as briefly noted above, if the desired distribution isnon-uniform, then the correction is the difference between the desireddistribution and n(r, z=wafer).

The next step (block 940) is to select a “test” current I_(i) for eachof the overhead coils 60, 65 and apply that current to the appropriatecoil and measure the resulting plasma ion distribution, which may bewritten n(r, z=wafer)_(test). The change in ion distribution Δn(r) isobtained by subtracting the ion distributions measured with and withoutthe magnetic field:Δn(r)≈n(r,z=wafer)−n(r,z=wafer)_(test)

The next step (block 950) is to compute a scale factor S relating thepressure gradient exerted by the magnetic field (i.e., the magneticpressure) to the change in ion distribution Δn(r). This computation isperformed by dividing the magnetic pressure gradient by Δn(r). Themagnetic pressure gradient of the magnetic field B(r, z=wafer, I_(i)) ofthe i^(th) coil is computed individually for each of the coils inaccordance with the magneto-hydrodynamics equation:∇_(r) P≈−∇ _(r) [B(r,z=wafer,I _(i))²/2μ₀]

where the subscript r denotes radial component. The results thusobtained for each coil individually are then summed together. Therefore,the total magnetic pressure gradient is:−∇_(r){Σ_(i) [B(r,z=wafer,I _(i))²/2μ₀]}Therefore, the scale factor S is:S={−∇ _(r){Σ_(i) [B(r,z=wafer,I _(i))²/2μ₀ ]}}/Δn(r)

This division operation may be carried out at different values of r andthe results averaged to obtain S in scalar form. Otherwise, the scalefactor S will be a function of r and used in the appropriate manner.

The scale factor S found in the step of block 950 is a link between thecoil currents I_(i) that determine the magnetic pressure and a resultingchange in ion distribution. Specifically, given a set of coil currentsI_(i), a corresponding change in ion distribution n(r) can be computedby multiplying the magnetic pressure determined from the set of I_(i) bythe scale factor S:Δn(r)={−∇_(r){Σ_(i) [B(r,z=wafer,I _(i))²/2μ₀ ]}}/S

This fact provides the basis for the following step (block 960) in whicha computer (such as the microprocessor 91) uses the foregoing equationto search for a set of coil currents I_(i) that produces the bestapproximation to previously specified or desired change in plasma iondensity distribution, Δn(r). In this case, the desired change is equalto the correction function c(r) computed in the step of block 930. Inother words, the computer searches for a set of coil currents I_(i) thatsatisfies the following condition:{−∇_(r){Σ_(i) [B(r,z=wafer,I _(i))²/2μ₀ ]}}=c(r)S

This search may be carried out by well-known optimization techniquesinvolving, for example, the method of steepest descents. Such techniquesare readily carried out by the worker skilled in this field and need notbe described here.

The magnitudes and polarities of the set of coil currents I_(i)discovered by the search are then sent to the controller 90, which inturn applies these currents to the respective coils 60, 65.

FIG. 10 compares magnetic pressure (solid line) with the measured changein plasma ion distribution (dotted line) as a function of radialposition at the wafer surface. As discussed above, the magnetic pressureis the gradient of the square of the magnetic fields of the overheadcoils. FIG. 10 indicates that there is good correlation between magneticpressure and change in ion density distribution.

The application of such a method is illustrated in FIGS. 11-14. FIG. 11illustrates how non-uniformity or the standard deviation (vertical axis)in the etch rate spatial distribution at the wafer surface varied withcoil current in one of the overhead coils. At zero coil current, thestandard deviation was about 12%, and the ion distribution wascenter-high as shown in FIG. 12.

The minimum non-uniformity at about 3% was achieved at a coil current ofabout 17 amperes. This represents an improvement by about a factor offour (i.e., 12% to 3% standard deviation in the etch rate distribution).The actual or measured etch rate distribution was as shown in FIG. 13A,while the etch rate distribution predicted using the techniques of FIG.9 was as shown in FIG. 13B.

At the high coil current of 35 amperes, the etch rate distributionstandard deviation was about 14%. The measured etch rate spatialdistribution was as shown in FIG. 14A while the predicted distributionwas as shown in FIG. 14B.

Referring again to FIG. 13A, the most uniform ion distribution obtainedis certainly not flat and in fact has “bowl” shape, being concave nearthe periphery and convex near the center. It is possible that with agreater number of independent overhead coils (e.g., three or more), theoptimization of currents may be carried out with greater resolution andbetter uniformity in results. Therefore, the invention is not limited tothe cases having only two coils. The invention may be implemented withvarying results using less than or more than two overhead coils.

The same method may be applied in order to control plasma ion densitydistribution or etch rate distribution at the ceiling surface. Such anapproach may be useful during chamber cleaning operations, for example.FIG. 15 illustrates a version of the method of FIG. 9 in whichuniformity of the spatial distribution of ion density (or, etch rate) isoptimized. The steps of FIG. 15, namely blocks 910′, 920′, 930′, 940′,950′ and 960′ are the same as the steps of FIG. 9, namely blocks 910,920, 930, 940, 950 and 960, except that they are carried out for theceiling plane rather than the wafer plane:

The first step (block 910′ of FIG. 15) is to analytically determine, foreach one of the overhead coils 60, 65, the expression for the magneticfield at the ceiling surface as a function of current flow in the coiland radial location on the wafer surface. Using cylindrical coordinates,this expression may be written, for the i^(th) coil, as B_(i)(r,z=ceiling, I_(i)). It is determined from simple static magnetic fieldequations and is a function not only of coil current I_(i) and radiallocation r on the ceiling surface but also of certain constants such asthe radius of the coil and the distance, z=ceiling, between the coil andthe ceiling interior surface.

The next step (block 920′ of FIG. 15) is carried out with no currentflowing in the overhead coils 60, 65. In this step, the spatialdistribution of plasma ion density across the ceiling surface ismeasured. This spatial distribution may be written as n(r, z=ceiling).In this step, the plasma ion density distribution can be measured by aconventional probe or other indirect techniques.

Next, in the step of block 930′, a correction, c′(r), to the measuredplasma ion density spatial distribution function n(r, z=ceiling)measured in the previous step is determined. (It should be noted thatthe prime notation ′ is employed here to distinguish the computations ofFIG. 15 from those of FIG. 9 described above, and does not connote aderivative as used herein.) The correction c′(r) may be defined in anynumber of appropriate ways. For example, it may be defined as themaximum value n(r, z=ceiling)_(max) minus n(r, z=ceiling). In this way,adding c′(r) to n(r, z=ceiling) produces a “corrected” distribution witha uniform amplitude equal to n(r)_(max). Of course, the correctionfunction c′(r) may be defined differently to produce a different uniformamplitude. Also, if a particular non-uniform distribution is desired,then the correction is the difference between the uncorrected or ambientplasma distribution n(r, z=ceiling) and the desired non-uniformdistribution. Thus, the method can be employed to establish either adesired plasma ion distribution having a particular non-uniform patternor to establish a uniform plasma ion density distribution.

The next step (block 940′) is to select a “test” current I_(i) for eachof the overhead coils 60, 65 and apply that current to the appropriatecoil and measure the resulting plasma ion distribution, which may bewritten n(r, z=ceiling)_(test). The change in ion distribution Δn(r) isobtained by subtracting the ion distributions measured with and withoutthe magnetic field:Δn′(r)=n(r,z=ceiling)−n(r,z=ceiling)_(test)

The next step (block 950′) is to compute a scale factor S′ relating thepressure gradient exerted by the magnetic field (i.e., the magneticpressure) to the change in ion distribution Δn′(r). This computation isperformed by dividing the magnetic pressure gradient by Δn′(r). Themagnetic pressure gradient of the magnetic field B(r, z=ceiling, I_(i))of the i^(th) coil is computed individually for each of the coils inaccordance with the magneto-hydrodynamics equation:∇_(r) P=−∇ _(r) [B(r,z=ceiling,I _(i))²/2μ₀]where the subscript r denotes radial component. The results thusobtained for each coil individually are then summed together. Therefore,the total magnetic pressure gradient is:−∇_(r){_(—) _(i) [B(r,z=wafer,I _(i))²/2μ₀]}Therefore, the scale factor S is:S′={−∇ _(r){Σ_(i) [B(r,z=wafer,I _(i))²/2μ₀ ]}}/Δn′(r)

The scale factor S′ found in the step of block 950′ is a link betweenthe coil currents I_(i) that determine the magnetic pressure and aresulting change in ion distribution. Specifically, given a set of coilcurrents I_(i), a corresponding change in ion distribution n′(r) can becomputed by multiplying the magnetic pressure determined from the set ofI_(i) by the scale factor S′:Δn′(r)={−∇_(r){Σ_(i) [B(r,z=wafer,I _(i))²/2μ₀ ]}}/S′

This fact provides the basis for the following step (block 960′) inwhich a computer (such as the microprocessor 91) uses the foregoingequation to search for a set of coil currents I_(i) that produces thebest approximation to previously specified or desired change in plasmaion density distribution, Δn′(r). In this case, the desired change isequal to the correction function c′(r) computed in the step of block930′. In other words, the computer searches for a set of coil currentsI_(i) that satisfies the following condition:{−∇_(r){Σ_(i) [B(r,z=wafer,I _(i))²/2μ₀ ]}}=c′(r)S′

This search may be carried out by well-known optimization techniquesinvolving, for example, the method of steepest descents. Such techniquesare readily carried out by the worker skilled in this field and need notbe described here.

The magnitudes and polarities of the set of coil currents I_(i)discovered by the search are then sent to the controller 90, which inturn applies these currents to the respective coils 60, 65.

With only a single overhead coil, the apparatus can be used to optimizeplasma ion distribution uniformity at either the wafer or the ceilingbut not both simultaneously. With at least two overhead coils (e.g., theoverhead coils 60 and 65), plasma ion distribution uniformity can be atleast approximately optimized at both the wafer and the ceilingsimultaneously.

Steering Plasma with the Overhead Coils:

We have discovered that the coil currents I_(i) may be selected in sucha manner as to steer the plasma toward the ceiling and/or side walls orto steer it to the wafer surface. The coil currents I_(i) may also beselected to improve uniformity of plasma density distribution at theceiling surface in a manner similar to the method of FIG. 9. As aresult, the plasma may be concentrated during processing on the wafer,and then during cleaning may be concentrated on the ceiling and/or sidewalls. By thus concentrating the plasma at the ceiling, cleaning timemay be reduced.

In one example, the plasma was steered to the side wall of the chamberby the controller 90 applying a current of −17.5 amperes to the innercoil 60 and a current of +12.5 amperes to the outer coil 65. FIG. 16illustrates a radial portion of the chamber interior extending along thehorizontal axis from zero radius to the periphery of the chamber andextending along the vertical axis from the wafer surface to the ceiling.The small arrows in FIG. 16 indicate the magnitude and direction of themagnetic field at various locations in the chamber when the plasma issteered to the side wall of the chamber by the controller 90 applying acurrent of −17.5 amperes to the inner coil 60 and a current of +12.5amperes to the outer coil 65. FIG. 17 illustrates the correspondinggradient of the square of the magnetic field at the wafer surface as afunction of radial position.

In another example, the plasma was steered to the roof of the chamber bythe controller 90 applying a current of −12.5 amperes to the inner coil60 and a current of +5 amperes to the outer coil 65. FIG. 18 illustratesa radial portion of the chamber interior extending along the horizontalaxis from zero radius to the periphery of the chamber and extendingalong the vertical axis from the wafer surface to the ceiling. The smallarrows in FIG. 18 indicate the magnitude and direction of the magneticfield at various locations in the chamber when the plasma is steered tothe side wall of the chamber by the controller 90 applying a current of−12.5 amperes to the inner coil 60 and a current of +5 amperes to theouter coil 65. FIG. 19 illustrates the corresponding gradient of thesquare of the magnetic field at the wafer surface as a function ofradial position.

In a further example, plasma was steered along field lines extendingfrom the center of the ceiling to the side wall by the controller 90applying a current of −25 amperes to the inner coil 60 and a current of+2.75 to the outer coil 65. FIG. 20 illustrates a radial portion of thechamber interior extending along the horizontal axis from zero radius tothe periphery of the chamber and extending along the vertical axis fromthe wafer surface to the ceiling. The small arrows in FIG. 20 indicatethe magnitude and direction of the magnetic field at various locationsin the chamber when the plasma is steered to the side wall of thechamber by the controller 90 applying a current of −25 amperes to theinner coil 60 and a current of +2.5 amperes to the outer coil 65. FIG.21 illustrates the corresponding gradient of the square of the magneticfield at the wafer surface as a function of radial position.

FIG. 17 shows that a high positive magnetic pressure on the plasma isexerted near the edge of the chamber when the plasma is steered to theedge. FIG. 19 shows that a low magnetic pressure on the plasma isexerted near the edge of the chamber when the plasma is directed to theedge of the ceiling. FIG. 21 shows that a high negative pressure ispresent near the chamber edge when the field lines extend from theceiling to the edge.

Thus, the currents in the overhead coils 60, 65 may be chosen to directthe plasma to various locations in the chamber that may requirecleaning, such as the ceiling and the side wall. Or, the plasma may beconcentrated more near the wafer. In order to steer the plasma to eitherthe wafer or the ceiling, or to apportion the plasma between the waferand the ceiling in accordance with some steering ratio SR, a method suchas that illustrated in FIG. 22 may be carried out.

Referring now to FIG. 22, the first step (block 2210 of FIG. 22) is todefine an analytical model of the magnetic field inside the chamber as afunction of all coil currents in the overhead coils (e.g., the pair ofcoils 60, 65). This is readily accomplished using static magnetic fieldequations by a worker skilled in this field, and need not be describedhere. The magnetic field is the sum of the individual magnetic fieldsfrom each of the coils. Each individual magnetic field is a function ofthe diameter of the respective coil, the location of each coil, thecurrent flow in the coil and the location in the chamber. Thus, themagnetic field produced by the i^(th) coil may be written as:B(x,y,z,I_(i))so that the total magnetic field is:Σ_(i){B(x,y,z,I_(i))}

The next step (block 2220) is to select a set of magnetic fields thatfulfill a set of desired process conditions. For example, to steerplasma to the ceiling, a magnetic field is selected that produces amagnetic pressure on the plasma that pushes the plasma toward theceiling, as illustrated in the example of FIG. 18. To steer the plasmatoward the side wall, a magnetic field is chosen that produces amagnetic pressure on the plasma that pushes the plasma toward theperiphery, as illustrated in FIG. 16.

For each magnetic field defined in the step of block 2220 above thatfulfills a particular condition, a computer searches the model definedin the step of block 2210 for a set of coil currents that produce thedesired magnetic field. This is the next step of block 2230. Each set ofcurrents found in the step of block 2230 is stored along with the nameof the corresponding condition in a memory location associated with thecorresponding process condition (block 2240 of FIG. 22). Whenever aparticular process condition is selected (e.g., steering the plasma tothe ceiling), then the microprocessor 91 fetches the set of currentvalues from the corresponding memory location (block 2250) and causesthe corresponding currents to be applied to the appropriate coils (block2260).

FIG. 23 shows how the microprocessor 91 may be programmed to respond touser inputs. A determination is first made whether the processingincludes etching of the wafer surface (block 2310 and whether theprocess includes cleaning (etching) the ceiling (block 2320). If onlythe wafer is to be etched, then the plasma is steered to the wafer(block 2330) and the plasma distribution uniformity at the wafer surfaceis optimized (block 2350) using the method of FIG. 9. If the wafer is toetched while the ceiling is to cleaned at the same time, then the plasmadensity is apportioned between the ceiling and the wafer (block 2360)and plasma density uniformity is optimized at the wafer surface as inFIG. 9 and at the ceiling as in FIG. 15 (block 2370). If only theceiling is to be cleaned, then the plasma is steered to the ceiling(block 2380) and plasma density uniformity at the ceiling is optimized(block 2390).

Use with VHF Overhead Electrode:

FIG. 24 illustrates how the inner and outer coils 60, 65 may be combinedwith a capacitively coupled reactor that has an overhead electrodeconnected to a VHF plasma source power generator through a fixed tuningstub. Such a reactor is described in U.S. patent application Ser. No.10/028,922 filed Dec. 19, 2001 by Daniel Hoffman et al. entitled “PlasmaReactor with Overhead RF Electrode Tuned to the Plasma” and assigned tothe present assignee, the disclosure of which is incorporated herein byreference.

Referring to FIG. 24, a plasma reactor includes a reactor chamber 100with a wafer support 105 at the bottom of the chamber supporting asemiconductor wafer 110. A process kit may include, in an exemplaryimplementation, a conductive or semi-conductive ring 115 supported by adielectric ring 120 on a grounded chamber body 127. The chamber 100 isbounded at the top by a disc shaped overhead conductive electrode 125supported at a gap length above the wafer 110 on grounded chamber body127 by a dielectric seal. In one implementation, the wafer support 105is movable in the vertical direction so that the gap length may change.In other implementations, the gap length may be a fixed predeterminedlength. The overhead electrode 125 may be a metal (e.g., aluminum) whichmay be covered with a semi-metal material (e.g., Si or SiC) on itsinterior surface, or it may be itself a semi-metal material. An RFgenerator 150 applies RF power to the electrode 125. RF power from thegenerator 150 is coupled through a coaxial cable 162 matched to thegenerator 150 and into a coaxial stub 135 connected to the electrode125. The stub 135 has a characteristic impedance, has a resonancefrequency, and provides an impedance match between the electrode 125 andthe coaxial cable 162 or the output of the RF power generator 150, aswill be more fully described below. The chamber body is connected to theRF return (RF ground) of the RF generator 150. The RF path from theoverhead electrode 125 to RF ground is affected by the capacitance ofthe dielectric seal 120 and by the capacitance of the dielectric seal130. The wafer support 105, the wafer 110 and the process kit conductiveor semiconductive ring 115 provide the primary RF return path for RFpower applied to the electrode 125.

As in the case of FIG. 1A, the inner coil 60 is less than half thediameter of the outer coil 65 and is in a plane farther away from thechamber than the outer coil 65. The outer coil 65 is located at or closeto the plane of the top of the electrode 125, while the inner coil 60 islocated well above the electrode 125. As in the case of FIG. 1A, theD.C. currents in the coils 60, 65 are controlled by the plasma steeringcontroller 90 governing the current supplies 70, 75 of the coils 60, 65.

The capacitance of the overhead electrode assembly 126, including theelectrode 125, the process kit 115, 120 and the dielectric seal 130measured with respect to RF return or ground was, in one exemplary case,180 pico farads. The electrode assembly capacitance is affected by theelectrode area, the gap length (distance between wafer support andoverhead electrode), and by factors affecting stray capacitances,especially the dielectric values of the seal 130 and of the dielectricring 120, which in turn are affected by the dielectric constants andthicknesses of the materials employed. More generally, the capacitanceof the electrode assembly 126 (an unsigned number or scalar) is equal ornearly equal in magnitude to the negative capacitance of the plasma (acomplex number) at a particular source power frequency, plasma densityand operating pressure, as will be discussed below.

Many of the factors influencing the foregoing relationship are in greatpart predetermined due to the realities of the plasma processrequirements needed to be performed by the reactor, the size of thewafer, and the requirement that the processing be carried out uniformlyover the wafer. Thus, the plasma capacitance is a function of the plasmadensity and the source power frequency, while the electrode capacitanceis a function of the wafer support-to-electrode gap (height), electrodediameter, and dielectric values of the insulators of the assembly.Plasma density, operating pressure, gap, and electrode diameter mustsatisfy the requirements of the plasma process to be performed by thereactor. In particular, the ion density must be within a certain range.For example, silicon and dielectric plasma etch processes generallyrequire the plasma ion density to be within the range of 10⁹ to 10¹²ions/cc. The wafer electrode gap provides an optimum plasma iondistribution uniformity for 8 inch wafers, for example, if the gap isabout 2 inches. The electrode diameter is preferably at least as greatas, if not greater than the diameter of the wafer. Operating pressuressimilarly have practical ranges for typical etch and other plasmaprocesses.

But it has been found that other factors remain which can be selected toachieve the above preferred relationship, particularly choice of sourcefrequency and choice of capacitances for the overhead electrode assembly126. Within the foregoing dimensional constraints imposed on theelectrode and the constraints (e.g., density range) imposed on theplasma, the electrode capacitance can be matched to the magnitude of thenegative capacitance of the plasma if the source power frequency isselected to be a VHF frequency, and if the dielectric values of theinsulator components of electrode assembly 126 are selected properly.Such selection can achieve a match or near match between source powerfrequency and plasma-electrode resonance frequency.

Accordingly in one exemplary case, for an 8-inch wafer the overheadelectrode diameter is approximately 11 inches, the gap is about 2inches, the plasma density and operating pressure is typical for etchprocesses as above-stated, the VHF source power frequency is 210 MHz(although other VHF frequencies could be equally effective), and thesource power frequency, the plasma electrode resonance frequency and thestub resonance frequency are all matched or nearly matched.

More particularly, these three frequencies are slightly offset from oneanother, with the source power frequency being 210 MHz, theelectrode-plasma resonant frequency being approximately 200 MHz, and thestub frequency being about 220 MHz, in order to achieve a de-tuningeffect which advantageously reduces the system Q. Such a reduction insystem Q renders the reactor performance less susceptible to changes inconditions inside the chamber, so that the entire process is much morestable and can be carried out over a far wider process window.

A currently preferred mode has chamber and pedestal diameters suitablefor accommodating a 12 inch diameter wafer, a wafer-to-ceiling gap ofabout 1.25 inch and an VHF source power frequency of 162 MHz (ratherthan the 210 MHz referred to above).

The coaxial stub 135 is a specially configured design which furthercontributes to the overall system stability, its wide process windowcapabilities, as well as many other valuable advantages. It includes aninner cylindrical conductor 140 and an outer concentric cylindricalconductor 145. An insulator 147 (denoted by cross-hatching in FIG. 24),having a relative dielectric constant of 1 for example, fills the spacebetween the inner and outer conductors 140, 145. The inner and outerconductors 140, 145 may be formed, for example, of nickel-coatedaluminum. In an exemplary case, the outer conductor 145 has a diameterof about 4 inches and the inner conductor 140 has a diameter of about1.5 inches. The stub characteristic impedance is determined by the radiiof the inner and outer conductors 140, 145 and the dielectric constantof the insulator 147. The stub 135 of the case described above has acharacteristic impedance of 65Ω. More generally, the stub characteristicimpedance exceeds the source power output impedance by about 20%-40% andpreferably by about 30%. The stub 135 has an axial length of about 29inches (a half wavelength at 220 MHz) in order to have a resonance inthe vicinity of 220 MHz to generally match while being slightly offsetfrom the VHF source power frequency of 210 MHz.

A tap 160 is provided at a particular point along the axial length ofthe stub 135 for applying RF power from the RF generator 150 to the stub135, as will be discussed below. The RF power terminal 150 b and the RFreturn terminal 150 a of the generator 150 are connected at the tap 160on the stub 135 to the inner and outer coaxial stub conductors 140, 145,respectively. These connections are made via a generator-to-stub coaxialcable 162 having a characteristic impedance that matches the outputimpedance of the generator 150 (typically, 50Ω) in the well-knownmanner. A terminating conductor 165 at the far end 135 a of the stub 135shorts the inner and outer conductors 140, 145 together, so that thestub 135 is shorted at its far end 135 a. At the near end 135 b (theunshorted end) of the stub 135, the outer conductor 145 is connected tothe chamber body via an annular conductive housing or support 175, whilethe inner conductor 140 is connected to the center of electrode 125 viaa conductive cylinder or support 176. A dielectric ring 180 is heldbetween and separates the conductive cylinder 176 and the electrode 125.

The inner conductor 140 provides a conduit for utilities such as processgases and coolant. The principal advantage of this feature is that,unlike typical plasma reactors, the gas line 170 and the coolant line173 do not cross large electrical potential differences. They thereforemay be constructed of metal, a less expensive and more reliable materialfor such a purpose. The metallic gas line 170 feeds gas outlets 172 inor adjacent the overhead electrode 125 while the metallic coolant line173 feeds coolant passages or jackets 174 within the overhead electrode125.

An active and resonant impedance transformation is thereby provided bythis specially configured stub match between the RF generator 150, andthe overhead electrode assembly 126 and processing plasma load,minimizing reflected power and providing a very wide impedance matchspace accommodating wide changes in load impedance. Consequently, wideprocess windows and process flexibility is provided, along withpreviously unobtainable efficiency in use of power, all while minimizingor avoiding the need for typical impedance match apparatus. As notedabove, the stub resonance frequency is also offset from ideal match tofurther enhance overall system Q, system stability and process windowsand multi-process capability.

Matching the Electrode-Plasma Resonance Frequency and the VHF SourcePower Frequency:

As outlined above, a principal feature is to configure the overheadelectrode assembly 126 for resonance with the plasma at theelectrode-plasma resonant frequency and for the matching (or the nearmatch of) the source power frequency and the electrode-plasma frequency.The electrode assembly 126 has a predominantly capacitive reactancewhile the plasma reactance is a complex function of frequency, plasmadensity and other parameters. (As will be described below in greaterdetail, a plasma is analyzed in terms of a reactance which is a complexfunction involving imaginary terms and generally corresponds to anegative capacitance.) The electrode-plasma resonant frequency isdetermined by the reactances of the electrode assembly 126 and of theplasma (in analogy with the resonant frequency of a capacitor/inductorresonant circuit being determined by the reactances of the capacitor andthe inductor). Thus the electrode-plasma resonant frequency may notnecessarily be the source power frequency, depending as it does upon theplasma density. The problem, therefore, is to find a source powerfrequency at which the plasma reactance is such that theelectrode-plasma resonant frequency is equal or nearly equal to thesource power frequency, given the constraints of practical confinementto a particular range of plasma density and electrode dimensions. Theproblem is even more difficult, because the plasma density (whichaffects the plasma reactance) and the electrode dimensions (which affectelectrode capacitance) must meet certain process constraints.Specifically, for dielectric and conductor plasma etch processes, theplasma density should be within the range of 10⁹-10¹² ions/cc, which isa constraint on the plasma reactance. Moreover, a more uniform plasmaion density distribution for processing 8-inch diameter wafers forexample, is realized by a wafer-to-electrode gap or height of about 2inches and an electrode diameter on the order of the wafer diameter, orgreater, which is a constraint on the electrode capacitance. On theother hand, a different gap may be utilized for a 12-inch diameterwafer.

Accordingly, by matching (or nearly matching) the electrode capacitanceto the magnitude of the negative capacitance of the plasma, theelectrode-plasma resonant frequency and the source power frequency areat least nearly matched. For the general conductor and dielectric etchprocess conditions enumerated above (i.e., plasma density between10⁹-10¹² ions/cc, a 2-inch gap and an electrode diameter on the order ofroughly 11 inches), the match is possible if the source power frequencyis a VHF frequency. Other conditions (e.g., different wafer diameters,different plasma densities, etc.) may dictate a different frequencyrange to realize such a match in carrying out this feature of thereactor. As will be detailed below, under favored plasma processingconditions for processing 8-inch wafers in several principalapplications including dielectric and metal plasma etching and chemicalvapor deposition, the plasma capacitance in one typical working examplehaving plasma densities as set forth above was between −50 and −400 picofarads. In an exemplary case the capacitance of the overhead electrodeassembly 126 was matched to the magnitude of this negative plasmacapacitance by using an electrode diameter of 11 inches, a gap length(electrode to pedestal spacing) of approximately 2 inches, choosing adielectric material for seal 130 having a dielectric constant of 9, anda thickness of the order of one inch, and a dielectric material for thering 120 having a dielectric constant of 4 and thickness of the order of10 mm.

The combination of electrode assembly 126 and the plasma resonates at anelectrode-plasma resonant frequency that at least nearly matches thesource power frequency applied to the electrode 125, assuming a matchingof their capacitances as just described. We have discovered that forfavored etch plasma processing recipes, environments and plasmas, thiselectrode-plasma resonant frequency and the source power frequency canbe matched or nearly matched at VHF frequencies; and that it is highlyadvantageous that such a frequency match or near-match be implemented.In an exemplary case, the electrode-plasma resonance frequencycorresponding to the foregoing values of plasma negative capacitance isapproximately 200 MHz, as will be detailed below. The source powerfrequency is 210 MHz, a near-match in which the source power frequencyis offset slightly above the electrode-plasma resonance frequency inorder to realize other advantages to be discussed below.

The plasma capacitance is a function of among other things, plasmaelectron density. This is related to plasma ion density, which needs, inorder to provide good plasma processing conditions, to be kept in arange generally 10⁹ to 10¹² ions/cc. This density, together with thesource power frequency and other parameters, determines the plasmanegative capacitance, the selection of which is therefore constrained bythe need to optimize plasma processing conditions, as will be furtherdetailed below. But the overhead electrode assembly capacitance isaffected by many physical factors, e.g. gap length (spacing betweenelectrode 125 and the wafer); the area of electrode 125; the range ofthe dielectric loss tangent for the dielectric seal 130; the choice ofdielectric constant of the dielectric seal 130 between electrode 125 andgrounded chamber body 127; the choice of dielectric constant for theprocess kit dielectric seal 130; and the thickness of the dielectricseals 130 and 120 and the thickness and dielectric constant of the ring180. This permits some adjustment of the electrode assembly capacitancethrough choices made among these and other physical factors affectingthe overhead electrode capacitance. We have found that the range of thisadjustment is sufficient to achieve the necessary degree of matching ofthe overhead electrode assembly capacitance to the magnitude of thenegative plasma capacitance. In particular, the dielectric materials anddimensions for the seal 130 and ring 120 are chosen to provide thedesired dielectric constants and resulting dielectric values. Matchingthe electrode capacitance and the plasma capacitance can then beachieved despite the fact that some of the same physical factorsinfluencing electrode capacitance, particularly gap length, will bedictated or limited by the following practicalities: the need to handlelarger diameter wafers; to do so with good uniformity of distribution ofplasma ion density over the full diameter of the wafer; and to have goodcontrol of ion density vs. ion energy.

Given the foregoing range for the plasma capacitance and the matchingoverhead electrode capacitance, the electrode-plasma resonance frequencywas approximately 200 MHz for a source power frequency of 210 MHz.

A great advantage of choosing the capacitance of the electrode assembly126 in this manner, and then matching the resultant electrode-plasmaresonant frequency and the source power frequency, is that resonance ofthe electrode and plasma near the source power frequency provides awider impedance match and wider process window, and consequently muchgreater immunity to changes in process conditions, and therefore greaterperformance stability. The entire processing system is rendered lesssensitive to variations in operating conditions, e.g., shifts in plasmaimpedance, and therefore more reliable along with a greater range ofprocess applicability. As will be discussed later in the specification,this advantage is further enhanced by the small offset between theelectrode-plasma resonant frequency and the source power frequency.

FIG. 25 illustrate how the inner and outer coils 60, 65 may be combinedwith a capacitively coupled reactor that has an overhead electrodeconnected to a VHF plasma source power generator through a fixed tuningstub, and has MERIE electromagnets around its periphery. Such a reactoris described in U.S. patent application Ser. No. 10/028,922 filed Dec.19, 2001 by Daniel Hoffman et al. entitled “Plasma Reactor with OverheadRF Electrode Tuned to the Plasma” and assigned to the present assignee,the disclosure of which is incorporated herein by reference.

Referring to FIG. 25, a VHF capacitively coupled plasma reactor includesthe following elements found in the reactor of FIG. 1A: a reactorchamber 100 with a wafer support 105 at the bottom of the chambersupporting a semiconductor wafer 110. A process kit in the illustratedcase consists of a semi-conductive or conductive ring 115 supported by adielectric ring 120 on the grounded chamber body 127. The chamber 100 isbounded at the top by a disc shaped overhead aluminum electrode 125supported at a predetermined gap length above the wafer 110 on groundedchamber body 127 by a dielectric seal 130. The overhead electrode 125also may be a metal (e.g., aluminum) which may be covered with asemi-metal material (e.g., Si or SiC) on its interior surface, or it maybe itself a semi-metal material. An RF generator 150 applies RF power tothe electrode 125. RF power from the generator 150 is coupled through acoaxial cable 162 matched to the generator 150 and into a coaxial stub135 connected to the electrode 125. The stub 135 has a characteristicimpedance, resonance frequency, and provides an impedance match betweenthe electrode 125 and the coaxial cable 162/RF power generator 150, aswill be more fully described below. The chamber body is connected to theRF return (RF ground) of the RF generator 150. The RF path from theoverhead electrode 125 to RF ground is affected by the capacitance ofthe process kit dielectric ring 120 and the dielectric seal 130. Thewafer support 105, the wafer 110 and the process kit semiconductive (orconductive) ring 115 provide the primary RF return path for RF powerapplied to the electrode 125.

As in the case of FIG. 1A, the inner coil 60 is less than half thediameter of the outer coil 65 and is in a plane farther away from thechamber than the outer coil 65. The outer coil 65 is located at or closeto the plane of the top of the electrode 125, while the inner coil 60 islocated well above the electrode 125. As in the case of FIG. 1, the D.C.currents in the coils 60, 65 are controlled by the plasma steeringcontroller 90 governing the current supplies 70, 75 of the coils 60, 65.

The improvement in plasma density distribution uniformity is achieved bythe introduction of a set of MERIE electromagnets 902 spaced equallyabout the periphery of the wafer support pedestal and outside of thereactor chamber (like those shown in FIGS. 7 and 8). These MERIE magnetsare adapted to produce a magnetic field that slowly rotates about theaxis of symmetry of the cylindrical chamber generally across the surfaceof the wafer support pedestal. In one case this feature is realized bythe MERIE magnets 902 having electromagnet windings wound aboutrespective axes tangent to the circumference of the wafer supportpedestal. In this case, an MERIE current controller 904 controls theindividual current to each MERIE magnet. A circulating magnetic field isgenerated in the plane of the workpiece support by the controller 904providing individual AC currents to each of the individual magnetwindings of the same frequency but offset in phase by 90 degrees (or by360 degrees divided by the number of MERIE magnets). In an alternativecase, the feature of a rotating magnetic field is realized by a supportframe 1020 (dashed line) supporting all of the MERIE magnets that isrotated about the axis of symmetry by a rotor 1025 (dashed line). Inthis alternative case, the MERIE magnets are permanent magnets.

A second array of MERIE magnets 906 (shown in dashed line) equallyspaced about the workpiece or wafer support pedestal but in a higherplane than the first set of MERIE magnets 902 may be provided as well.Both sets of magnets lie in respective planes that are near the plane ofthe workpiece support.

The controller 904 applies a low frequency (0.5-10 Hz) AC current toeach of the electromagnets 902, 906, the phases of the currents appliedto neighboring magnets being offset as described above by 90 degrees.The result is a magnetic field that rotates about the axis of symmetryof the workpiece support at the low frequency of the AC current. Themagnetic field causes the plasma to be drawn toward the magnetic fieldnear the workpiece surface and to circulate with the field. This stirsthe plasma so that its density distribution becomes more uniform. As aresult, reactor performance is significantly improved because moreuniform etch results are obtained across the entire surface of thewafer.

Combination Overhead Electrode and Gas Distribution Plate:

It is desirable to feed the process gas from the overhead ceiling toimprove uniformity of gas distribution within the chamber. For thispurpose, the overhead electrode 125 in the cases of FIGS. 24 and 25 canbe a gas distribution showerhead, and therefore has a large number ofgas injection ports or small holes 300 in its bottom surface facing theworkpiece support 105. In an exemplary case, the holes 300 were between0.01 and 0.03 inch in diameter and their centers were uniformly spacedapart by about ⅜ inch.

The overhead electrode/gas distribution plate 125 (hereinafter referredto as the gas distribution plate 125) has improved resistance to arcing.This is due to the introduction of an arc suppression feature thatexcludes process gas and/or plasma from the center of each opening orhole 300. This arc suppressing feature is a set of center pieces ordisks 302 in the centers of the holes 300 supported at the ends ofrespective cylindrical fingers or thin rods 303 as shown in thecross-sectional view of FIG. 26 and the enlarged cross-sectional view ofFIG. 27. Arcing within a typical gas distribution plate tends to occurnear the center of the gas injection holes. Therefore, placing thecenter pieces 302 at the center of each hole 300 prevents process gasfrom reaching the center of each hole 300 and therefore reduces theoccurrence of arcing. As shown in the plan view of FIG. 28, introductionof the center pieces 302 in the holes 300 transforms the otherwisecircular openings or holes 300 into annular openings.

Referring to FIG. 29A, the gas distribution plate 125 with improved arcsuppression constitutes a cover 1402 and a base 1404. The base 1404 is adiscoid plate 1406 with the gas injection openings formed therethroughsurrounded by an annular wall 1408 having an interior shoulder 1410. Thecover 1402 is also a discoid plate. The disks 302 are the end sectionsof the cylindrical fingers 303 attached to and extending downwardly fromthe bottom surface of the cover 1402. The outer edge of the cover 1402rests on the shoulder 1410 of the base 1404 to form a gas manifold 1414(FIG. 26) between the cover 1402 and the base 1404. Process gas flowsinto the manifold 1414 from a gas inlet 1416 in the center of the cover1402.

The portions of the gas distribution plate 125 that contact process gasor plasma in the chamber can be formed of a metal such as aluminumcoated with a semiconductor processing compatible material such assilicon carbide. In this example, all surfaces of the gas distributionplate, with the exception of the top surface of the cover 1402, arecovered with a silicon carbide coating 1502 as indicated in the enlargedpartial cross-sectional view of FIG. 29B. As shown in FIG. 30, thealuminum top surface of the cover 1402 is in contact with atemperature-controlled member 1520 that may be water-cooled by waterjackets 1522 with coolant circulated by a heat exchanger 1524, so thatthe thermally conductive aluminum material of the gas distribution plate125 has a controlled temperature. Alternatively, as shown in FIG. 31,the water jackets may be within the gas distribution plate 125.

However, in order for the silicon carbide coating 1502 to have the samecontrolled temperature, there must be a thermally conductive bondbetween the silicon carbide coating and the aluminum. Otherwise, thetemperature of the silicon carbide coating could fluctuateuncontrollably. In order to achieve good thermal conductivity betweenthe aluminum material of the gas distribution plate 125 and the siliconcarbide coating, a polymer bonding layer 1504 is formed between thealuminum gas distribution plate and the silicon carbide coating 1502, asshown in FIG. 29A. FIG. 29A shows that the polymer bonding layer 1504 isbetween the silicon carbide coating 1502 and the aluminum base 1404. Thepolymer bonding layer provides good thermal conductivity between thealuminum and the silicon carbide coating 1502, so that the temperatureof the coating 1502 is controlled by the heat exchanger 1524.

FIGS. 32, 33 and 34 illustrate how the gas distribution plate 125 ofFIG. 29A can be modified to provide dual zone gas flow control. Such afeature can be employed to help correct an etch rate or deposition ratespatial distribution that is either center high or center low byselecting a process gas distribution that is complementary.Specifically, an annular partition or wall 1602 divides the gas manifold1414 into a center manifold 1414 a and an outer manifold 1414 b. Inaddition to the center gas feed 1416 that feeds the center manifold 1414a, another gas feed 1418 between the center and periphery of the gasdistribution plate 125 feeds the outer manifold 1414 b. A dual zonecontroller 1610 apportions gas flow from a process gas supply 1612between the inner and outer gas feeds 1416, 1418. FIG. 35 illustratesone implementation of the valve 1610 in which an articulating vane 1618controls the relative amount of gas flow to the inner and outermanifolds 1414 a, 1414 b of the gas distribution plate. An intelligentflow controller 1640 governs the position of the vane 1618. In anotherimplementation illustrated in FIG. 36, a pair of valves 1651, 1652perform individual gas flow control for respective radial zones of thechamber.

FIG. 37 illustrates an case in which the gas distribution plate 125 hasthree gas flow zones, the manifold 1414 being separated by inner andouter annular partitions 1604, 1606 into three manifolds 1414 a, 1414 band 1414 c. Three respective gas feeds 1416, 1418, 1420 provide gas flowto the respective manifolds 1414 a, b, c.

While various cases have been described above in this specification ashaving a pair of overhead coils 60, 65, FIG. 37 shows that there can bemore than two overhead coils. In fact, the case of FIG. 37 isillustrated as having three concentric overhead coils or coils 60, 64and 65. By increasing the number of independently controlled overheadcoils, it is felt the resolution with which processing non-uniformitiesare corrected is increased.

The multiple zone gas distribution plates of FIGS. 34 and 37 enjoy theadvantage of flexible control over gas apportionment between inner andouter processing zones of the workpiece. However, another way ofcustomizing gas flow is to do so permanently by providing different gasinjection hole sizes at different radii of the gas distribution plate125. For example, if the reactor tends to exhibit a spatial etch ratedistribution that is center high, then less gas would be supplied nearthe center and more at the periphery of the chamber by using smaller gasinjection holes 300 at the center and larger ones near the periphery.Such a gas distribution plate is illustrated in plan view in FIG. 38.For a center low etch distribution, the opposite hole arrangement wouldbe employed as illustrated in FIG. 39.

Plasma Steering in the Reactor of FIG. 9:

Plasma steering as described above with reference to FIGS. 11-14 wasperformed in the case of FIG. 9. A magnetic field pointing to the sidewall was produced by applying a current of −13 amperes to the inner coil60 and a current of +1.4 amperes to the outer coil 65. A magnetic fieldpointing toward the periphery of the ceiling or electrode 125 wasproduced by applying a current of −13 amperes to the inner coil 60 and acurrent of +5.2 amperes to the outer coil 65. A dense magnetic field atthe side wall was produced by applying a current of −13 amperes to theinner coil 60 and a current of +9.2 amperes to the outer coil 65. Wefound that the etch rate of chamber surfaces during cleaning wereimproved by as much as 40% by applying a magnetic field pointing towardthe periphery of the ceiling or electrode 125 in the manner describedabove.

Coil Configurations:

While the foregoing cases have been described with reference to theinner and outer coils 60, 65, a greater number of coils may be employed.For example, the case of FIG. 40 has five overhead coils 4060, 4062,4064, 4066, 4068, each with its own current separately controlled by thecontroller 90. The coils 4060, 4062, 4064, 4066, 4068 may be at the sameheight above the ceiling 125 (as in FIG. 40) or at different heights.FIG. 41 illustrates an case in which the overhead coils 60, 65 are atthe same height. In FIG. 41, the windings in each coil 60, 65 arestacked in both vertical and radial directions. FIGS. 42 and 43illustrate different cases in which the coils 60, 65 have windingsextending in the vertical direction and in the radial direction.

As discussed previously in this specification with reference to FIG. 1A,magnetic pressure on the plasma for correcting non-uniform distributionis proportional to the radial component of the gradient of the square ofthe magnetic field. Thus, the most efficient approach is to employ amagnetic field having a large radial gradient, such as a cusp-shapedmagnetic field. As further discussed above, the greater efficiency ofthe cusp-shaped magnetic field reduces the required strength of themagnetic field for a given amount of magnetic pressure, thereby reducingor eliminating device damage associated with high magnetic fields. FIG.44 illustrates an case in which a fully cusp-shaped magnetic field isproduced by a pair of coils 4420, 4440 located above and below thechamber, respectively. Current flow in the top and bottom coils 4420,4440 is clockwise and counter-clockwise, respectively. FIG. 45 is asimplified illustration of the magnetic field line pattern of the fullycusp-shaped magnetic field produced by the pair of coils 4420, 4440.

FIG. 46 illustrates an case in which the four electromagnets 4610, 4620,4630, 4640 of a conventional MERIE reactor 4650 are employed to generatethe fully cusp-shaped magnetic field of FIG. 45. A current controller4660 controlling the currents in each of the electromagnets 4610, 4620,4630, 4640 is programmed to apply D.C. currents flowing in the same(e.g., clockwise) direction in all the electromagnets 4610, 4620, 4630,4640, as indicated by the arrows in FIG. 46. In this way the D.C.currents in the top conductors 4610 a, 4620 a, 4630 a, 4640 a form aclockwise current loop, the D.C. currents in the bottom conductors 4610b, 4620 b, 4630 b, 4640 b form a counter-clockwise current loop, whileat each corner of the array the currents in the vertical conductors ofadjacent electromagnets (e.g., the pair of vertical conductors 4620 cand 4630 d) cancel the magnetic fields of one another at the wafersurface. The net effect is to produce clockwise and counter-clockwisecurrent loops at the top and bottom of the chamber, respectively,analogous to the case of FIG. 44, with the same resulting fullycusp-shaped magnetic field illustrated in FIG. 45. The reactor of FIG.46 is operated in any one of three modes:

magnetic pressure mode, in which the cusp-shaped field is produced;

sine wave mode, in which four sine wave currents are applied inquadrature to the four electromagnets 4610, 4620, 4630, 4640 to producea slowly rotating magnetic field over the wafer surface;

configurable magnetic field (CMF) mode, in which the four electromagnets4610, 4620, 4630, 4640 are grouped into to opposing sets of adjacentpairs, one pair having one D.C. current and the opposite pair having theopposite D.C. current, to produce generally straight magnetic fieldlines extending across the wafer surface in a diagonal directionrelative to the orientation of the four electromagnets 4610, 4620, 4630,4640. This grouping is rotated by switching the currents so that themagnetic field rotates through four diagonal orientations. A timesequence of these orientations are illustrated in FIGS. 47A, 47B, 47Cand 47D.

In FIG. 47A, the electromagnets 4610, 4620 have a positive D.C. currentflow while the electromagnets 4630, 4640 have negative D.C. currentflow, and the resulting average magnetic field direction is generallyfrom the upper left corner to the lower right corner of the drawing. InFIG. 47B, the groupings have been switched so that the electromagnets4620, 4630 have the positive current flow while the electromagnets 4640,4610 have the negative current flow, and the average magnetic field hasrotated clockwise by 90 degrees. FIGS. 47C and 47D complete the cycle.The strength of the magnetic field lines is determined by the magnitudedifference in the positive and negative D.C. currents thus applied, andmay be adjusted by programming the controller 4650 as desired.

The method of FIG. 9 may be employed in the CMF mode to accuratelyselect the D.C. currents of the four electromagnets 4610, 4620, 4630,4640 to produce the best correction for non-uniform etch rate or plasmaion density distribution. In applying the method of FIG. 9 to the CMFmode of FIGS. 47A-D, the coils of each of the electromagnets or coils4610, 4620, 4630, 4640 are substituted for the overhead coils 60, 65,and all steps of FIG. 9 are performed in accordance with thatsubstitution. The only difference is that the calculation of themagnetic field from each coil is computed as an average over the fourtime periods corresponding to FIGS. 47A-D.

FIG. 48 illustrates a reactor including a special grating 4810 insertedover the pumping annulus. The grating 4810 is formed of a semiconductivematerial such as silicon carbide or of a conductive material such asaluminum and has openings 4820 for permitting gas to be evacuated fromthe chamber through the pumping annulus. The special grating 4810excludes plasma from the pumping annulus, providing needed protectionand process control. For this purpose, the distance across the interiorof each opening 4820 in the radial plane is no greater than twice theplasma sheath thickness. In this way it very difficult if not impossiblefor a plasma to penetrate through the grating 4810. This reduces oreliminates plasma interaction with chamber surfaces within the pumpingannulus.

FIGS. 49 and 50 illustrate an integrally formed removable chamber liner4910 that incorporates the plasma-confining grating 4810 of FIG. 48. Theliner 4910 covers the portions of the chamber that are radially outsideof the region underlying the electrode 125 and overlying the wafer 110.Thus, the liner 4910 includes an upper horizontal section 4920 coveringan outer periphery of the chamber ceiling, a vertical section 4930covering the chamber side wall and a lower horizontal section 4940 thatincludes the plasma-confining grating 4810 and covers the pumpingannulus as well as an annular surface adjacent the wafer 110. In onecase, each of the sections 4920, 4930, 4940 are formed together as amonolithic silicon carbide piece 4950. The liner 4910 further includesan aluminum base 4960 underlying the lower horizontal section 4940 ofthe silicon carbide piece 4950 and is bonded thereto. The aluminum base4960 includes a pair of downwardly extending annular rails 4962, 4964that are relatively long and thin and provide good electricalconductivity to grounded structural elements of the chamber below thewafer support pedestal 105.

The reactor can have temperature control elements 4972, 4974 in thermalcontact with the downwardly extending annular rails 4962, 4964 as wellas a temperature control element 4976 in thermal contact with thevertical side section 4930. Each of the thermal control elements 4972,4974, 4976 can include cooling apparatus including coolant passages andheating apparatus including an electric heater. It can be desirable tomaintain the liner 4910 at a sufficiently high temperature (e.g., ashigh as 120 degrees F.) to minimize or prevent deposition of polymer orfluorocarbon compounds on interior surfaces of the liner 4910.

The liner 4910 enhances process stability because it provides a goodground return path. This is due to the fact that the electric potentialis uniform along the interior surface of the silicon carbide piece 4950(including the interior-facing surfaces of the upper horizontal section4920, the vertical section 4930 and the lower horizontal section 4940).As a result, the liner 4910 provides a uniform RF return path at all ofits interior-facing surfaces for power delivered either from theoverhead electrode 125 or from the wafer pedestal 105. One advantage isthat as plasma fluctuations move the RF return current distribution toconcentrate at different parts of the interior surface of the liner4910, the impedance presented to that current remains fairly constant.This feature promotes process stability.

FIG. 51 illustrates a modification of the case of FIG. 7 in which theoverhead solenoids 60, 65 define a square pattern symmetrical with thesquare pattern of the MERIE magnets 92, 94, 96, 98, and is particularlysuited for uniform processing of a square semiconductor or dielectricworkpiece 4910, such as a photolithographic mask.

FIG. 52 illustrates a version of the reactor of FIG. 24 in which thewafer support pedestal 105 may be moved up and down. In addition to thetwo overhead coils 60, 65 for controlling plasma ion radialdistribution, there is a bottom coil 5210 below the plane of the wafersupport pedestal 105. In addition, there is an outer coil 5220 at theperiphery of the chamber. The outer overhead coil 65 and the bottom coil5210 can have opposing D.C. currents to form a full cusp magnetic fieldwithin the chamber.

While the overhead coils 60, 65 have been described in combination withreactor having an overhead ceiling that serves as both an overheadsource power electrode and as a gas distribution plate, the ceiling maybe of the type that is not a gas distribution plate, with process gasesbeing introduced in another conventional fashion (e.g., through the sidewall). Moreover, the coils 60, 65 may be employed in a reactor in whichsource power is not capacitively coupled by a ceiling electrode. Also,the impedance match element for the overhead electrode has beendescribed as being a fixed element such as a coaxial tuning stub.However, the impedance match element may be any suitable or conventionalimpedance match device such as a conventional dynamic impedance matchcircuit.

While the invention has been described in detail by specific referenceto preferred cases, it is understood that variations and modificationsthereof may be made without departing from the true spirit and scope ofthe invention.

1. A wafer processing apparatus, comprising: a housing defining aprocess chamber a wafer support configured to support a wafer within thechamber during processing; a first process gas inlet; a second processgas inlet; a gas distribution system, comprising: a center circular gasdisperser configured to receive a process gas from the first process gasinlet and to distribute the process gas into the chamber over the waferthrough a first plurality of injection ports; and an outer annular gasdisperser centered around the center gas disperser configured to receivethe process gas from the second process gas inlet and to distribute theprocess gas into the chamber over the wafer through a second pluralityof injection ports.
 2. The apparatus of claim 1 wherein the first andsecond plurality of injection ports are annular.
 3. The apparatus ofclaim 1 wherein the first and second plurality of injection ports arecircular holes, wherein the holes have diameters ranging between 0.01and 0.03 inches.
 4. The apparatus of claim 1 wherein the gasdistribution system comprises an annular wall that separates the centergas disperser and the outer gas disperser.
 5. The apparatus of claim 1further comprising: a first gas flow controller coupled to the firstprocess gas inlet, wherein the first gas flow controller can beindependently controlled to adjust the amount of the process gas flowinginto the center gas disperser; a second gas flow controller coupled tothe second process gas inlet; and wherein the first and the second gasflow controllers can be independently controlled to adjust the amount ofthe process gas flowing into the center circular gas disperser relativeto the amount of process gas flowing into the outer annular gasdisperser.
 6. The apparatus of claim 5 wherein the first gas flowcontroller comprises a first valve and wherein the second gas flowcontroller comprises a second valve.
 7. The apparatus of claim 5 furthercomprising a dual zone controller coupled to the first gas flowcontroller and the second gas flow controller, the dual zone controllerconfigured to adjust flow through the first gas flow controller andthrough the second gas flow controller.
 8. A wafer processing apparatus,comprising: a housing defining a processing chamber, the housing coupledto an RF ground; a substrate support located in a chamber configured tosupport a wafer during processing; first and second process gas inletsconfigured to deliver a process gas into the chamber; a gas distributionsystem comprising a circular gas disperser having a circular center gasdispersing region fluidly coupled to the first process gas inlet and anannular gas dispersing region surrounding the center region and fluidlycoupled to the second process gas inlet, wherein the center gasdispersing region comprises a first plurality of gas injection holesconfigured to introduce the process gas into the chamber above a wafersupported on the substrate support and the annular gas dispersing regioncomprises a second plurality of gas injection holes configured tointroduce the process gas into the chamber annularly to the center gasdispersion region above the wafer; and an RF generator coupled to animpedance match circuit used to provide RF power to the wafer support,wherein the impedance match circuit is coupled to the wafer support andwherein the RF generator is coupled to the RF ground.
 9. The apparatusof claim 8 wherein the first and second plurality of gas injectionsholes are annular.
 10. The apparatus of claim 8 wherein the first andsecond plurality of injection holes have diameters ranging between 0.01and 0.03 inches.
 11. The apparatus of claim 8 wherein the gasdistribution system comprises an annular wall that forms a boundaryseparating the center gas disperser and the outer gas disperser.
 12. Theapparatus of claim 8 wherein the first plurality of gas injection holesare configured to introduce the process gas into a center portion of awafer supported on the substrate support and the second plurality of gasinjection holes are configured to introduce the process gas into thechamber above an outer peripheral portion of the wafer.
 13. Theapparatus of claim 8 further comprising: a first gas flow controllercoupled to the first process gas inlet, wherein the first gas flowcontroller can be independently controlled to adjust the amount of theprocess gas flowing into the center gas disperser; and a second gas flowcontroller coupled to the second process gas inlet, wherein the secondgas flow controller can be independently controlled to adjust the amountof the process gas flowing into the outer annular gas disperser.
 14. Theapparatus of claim 13 further comprising a dual zone controller coupledto the first gas flow controller and the second gas flow controller, thedual zone controller configured to adjust flow through the first gasflow controller and through the second gas flow controller.
 15. Theapparatus of claim 8 further comprising an annular pumping channel belowand surrounding the wafer support coupled to an exhaust line.
 16. Awafer processing apparatus, comprising: a vacuum chamber configured tosupport a plasma; a process gas inlet configured to deliver a processgas used for the plasma into the vacuum chamber; a gas disperser coupledto the process gas inlet, comprising: a base having a plurality ofinjection ports formed throughout, surrounded by an annular wall havingan interior shoulder; a cover having a top surface, a bottom surface anda plurality of fingers, the plurality of fingers attached to the bottomsurface and extending downwardly from the bottom surface, the topsurface coupled to the process gas inlet; and wherein the fingers extendinto the injection ports of the base to form a plurality of annularports in the base for the process gas to flow from the gas disperser toa processing region.
 17. The apparatus of claim 16 wherein the pluralityof injection ports are circular holes, wherein the holes have diametersranging between 0.01 and 0.03 inches.
 18. The apparatus of claim 17further comprising an annular wall positioned between the base and thecover forming a center gas disperser and an outer gas disperser.
 19. Theapparatus of claim 18 further comprising a second process gas inletcoupled to the top surface of the cover and positioned to be over theouter gas disperser.
 20. The apparatus of claim 19 further comprising: afirst gas flow controller coupled to the process gas inlet, wherein thefirst gas flow controller can be independently controlled to adjust theamount of the process gas flowing into the center gas disperser; and asecond gas flow controller coupled to the second process gas inlet,wherein the second gas flow controller can be independently controlledto adjust the amount of the process gas flowing into the outer gasdisperser.
 21. The apparatus of claim 19 further comprising a dual zonecontroller coupled to the first gas flow controller and the second gasflow controller, the dual zone controller configured to adjust flowthrough the first gas flow controller and through the second gas flowcontroller.